1. Field of the Invention
This invention relates to a method of producing a high strength silicon nitride sintered body.
2. Description of the Prior Art
Si.sub.3 N.sub.4 has high strength and is excellent in heat resistance and corrosion resistance, and therefore, it is expected to be utilized in wide applications such as cutting tools, engine materials for automobiles, materials for aerospace applications and the like.
Incidentally, Si.sub.3 N.sub.4 is difficult to sinter, because of strong covalent bonding properties, and methods to obtain a high density sintered body thereof are being developed in two courses. In one course, a small amount of an oxide such as Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, MgO or ZrO.sub.2 is added to Si.sub.3 N.sub.4 powder as a sintering aid (Yogyo Kyokai-shi, 94, 1, 106, 1986), and in the other course, a molded body without a sintering aid is sintered at a high temperature under a high pressure (Yogyo Kyokai-shi, 95, 2, 229 1987, Zairyo, 31, 349, 967).
According to the former method, the sintering can be conducted even at ordinary pressure. However, it is difficult to blend the sintering aid uniformly, and the sintering aid is liable to deposit at grain boundaries to induce the a decrease of strength. Besides, the sintering aid is softened at a high temperature, and therefore, high temperature strength sharply decreases particularly at higher than 1200.degree. C. In order to solve the above problems, several measures have been proposed such as the selection of the sintering aid (Yogyo Kyokai-shi, 93, 12, 796, 1986) and crystallization of grain boundaries (J. Mater. Sci., 21, 1345-1348, 1986), but these measures are still not sufficient.
In the latter method, the sintering is conducted by hot isostatic pressing (HIP) at 1000.degree. to 2200.degree. C. at 1000 to 2000 atm or by sintering at 1600.degree. to 2000.degree. C. at tens of thousands to hundreds of thousands atm. However, the above methods require very high temperature because of the absence of a sintering aid. Moreover, absolute strength is still low, and therefore, the reliability as materials is insufficient. The sintering under a very high pressure is undesirable from an industrial viewpoint, because it requires a large apparatus and restricting the form and size of sintered bodies.
Thus, a method of producing a high density silicon nitride sintered body at a low temperature without a sintering aid has not been developed yet.